The invention relates to a waveform generator, and particularly one in which a network composed of a series resonant circuit in series with a parallel resonant circuit is connected to a square wave voltage generator and the load can be coupled to the parallel resonant circuit, and wherein the resonant frequency of the parallel resonant circuit is essentially equal to the basic frequency of the square wave voltage generator and smaller than the resonant frequency of the series resonant circuit.
A sinusoidal output generator of this type has been proposed in German Offenlegungsschrift No. 26 48 758 for use in an induction heating apparatus. Induction heating apparatus are based on the principle of the formation of eddy currents in an electrically conductive body to be heated, which is in the alternating magnetic field of the coil of the parallel resonant circuit. The square wave voltage generator impresses a square wave voltage on the series connection of parallel resonant circuit and series resonant circuit, while the flow of the generator current as a function of time is determined by the resonant circuits. The parallel resonant circuit is adjusted to the basic or fundamental frequency of the square wave voltage, so that, during no-load operation of the generator, the basic frequency component of the generator current and, thus, the no-load losses caused by this current component are kept low.
However, in addition to the sine wave basic component of the generator current, the square wave voltage generates a plurality of harmonic waves. However the resistance of the parallel resonant circuit is low for the currents of the harmonic waves, so that relatively high losses occur in the known circuit even in the case of no-load. To keep the no-load losses caused by the harmonics as low as possible, the series resonant circuit is adjusted to a frequency which is slightly above the resonant frequency of the parallel resonant circuit. In this manner, there is little influence on the basic frequency component of the generator current, while its harmonic waves, particularly its third harmonic, are suppressed.
The known generator has the disadvantage that, although the no-load losses can be reduced, the efficiency of the generator is low and the power drain in the square wave voltage generator during operation is relatively high because of the misadjustment between the square wave voltage generator and the two resonant circuits which are connected in series.